<script id="background-shader-vs" type="x-shader/x-vertex">
    precision mediump float;

    attribute vec2 inPos;
    varying   vec2 vertPos;

    void main()
    {
        vertPos     = inPos;
        gl_Position = vec4( inPos, 0.0, 1.0 );
    }
</script>

<script id="background-shader-fs" type="x-shader/x-fragment">
    precision mediump float;

    varying vec2      vertPos;
    uniform sampler2D u_texture;
    
    void main()
    {
        vec2 texCoord = vec2( vertPos.s, -vertPos.t ) * 0.5 + 0.5;
        vec3 texColor = texture2D( u_texture, texCoord.st ).rgb;
        gl_FragColor  = vec4( texColor.rgb, 1.0 );
    }
</script>  

<script id="draw-shader-vs" type="x-shader/x-vertex">
precision mediump float;

attribute vec3 inPos;
attribute vec3 inCol;

varying vec3 vertCol;

uniform mat4 u_projectionMat44;
uniform mat4 u_viewMat44;
uniform mat4 u_modelMat44;

void main()
{
    vertCol       = inCol;
    vec4 modelPos = u_modelMat44 * vec4( inPos, 1.0 );
    vec4 viewPos  = u_viewMat44 * modelPos;
    gl_Position   = u_projectionMat44 * viewPos;
}
</script>

<script id="draw-shader-fs" type="x-shader/x-fragment">
precision mediump float;

varying vec3 vertCol;

void main()
{
    gl_FragColor = vec4( vertCol.rgb, 1.0 );
}
</script>

<style>
html,body { margin: 0; overflow: hidden; }
#gui { position : absolute; top : 0; left : 0; font-size : large; }
#ref-link { position : absolute; bottom : 0; left : 0; font-size : large; }  
</style>

<body onload="sceneStart();">

<canvas id="draw-canvas" style="border: none;" width="100%" height="100%"></canvas>

<a id="ref-link" href="https://stackoverflow.com/questions/48124131/how-can-i-draw-a-fullscreen-background-image-in-webgl-like-sketchfab/48128750#48128750">
<font color= #CCF>How can I draw a fullscreen background image in WebGL like sketchfab?</font> 
</a>

</body>

<script type="text/javascript">

glArrayType = typeof Float32Array !="undefined" ? Float32Array : ( typeof WebGLFloatArray != "undefined" ? WebGLFloatArray : Array );

function IdentM44() {
  var m = new glArrayType(16);
  m[0]  = 1; m[1]  = 0; m[2]  = 0; m[3]  = 0;
  m[4]  = 0; m[5]  = 1; m[6]  = 0; m[7]  = 0;
  m[8]  = 0; m[9]  = 0; m[10] = 1; m[11] = 0;
  m[12] = 0; m[13] = 0; m[14] = 0; m[15] = 1;
  return m;
};

function RotateAxis(matA, angRad, axis) {
    var aMap = [ [1, 2], [2, 0], [0, 1] ];
    var a0 = aMap[axis][0], a1 = aMap[axis][1]; 
    var sinAng = Math.sin(angRad), cosAng = Math.cos(angRad);
    var matB = new glArrayType(16);
    for ( var i = 0; i < 16; ++ i ) matB[i] = matA[i];
    for ( var i = 0; i < 3; ++ i ) {
        matB[a0*4+i] = matA[a0*4+i] * cosAng + matA[a1*4+i] * sinAng;
        matB[a1*4+i] = matA[a0*4+i] * -sinAng + matA[a1*4+i] * cosAng;
    }
    return matB;
}

function Cross( a, b ) { return [ a[1] * b[2] - a[2] * b[1], a[2] * b[0] - a[0] * b[2], a[0] * b[1] - a[1] * b[0], 0.0 ]; }
function Dot( a, b ) { return a[0]*b[0] + a[1]*b[1] + a[2]*b[2]; }
function Normalize( v ) {
    var len = Math.sqrt( v[0] * v[0] + v[1] * v[1] + v[2] * v[2] );
    return [ v[0] / len, v[1] / len, v[2] / len ];
}

var Camera = {};
Camera.create = function() {
    this.pos    = [0, 5, 0.0];
    this.target = [0, 0, 0];
    this.up     = [0, 0, 1];
    this.fov_y  = 90;
    this.vp     = [800, 600];
    this.near   = 0.5;
    this.far    = 100.0;
}
Camera.Perspective = function() {
    var fn = this.far + this.near;
    var f_n = this.far - this.near;
    var r = this.vp[0] / this.vp[1];
    var t = 1 / Math.tan( Math.PI * this.fov_y / 360 );
    var m = IdentM44();
    m[0]  = t/r; m[1]  = 0; m[2]  =  0;                              m[3]  = 0;
    m[4]  = 0;   m[5]  = t; m[6]  =  0;                              m[7]  = 0;
    m[8]  = 0;   m[9]  = 0; m[10] = -fn / f_n;                       m[11] = -1;
    m[12] = 0;   m[13] = 0; m[14] = -2 * this.far * this.near / f_n; m[15] =  0;
    return m;
}
Camera.LookAt = function() {
    var mz = Normalize( [ this.pos[0]-this.target[0], this.pos[1]-this.target[1], this.pos[2]-this.target[2] ] );
    var mx = Normalize( Cross( this.up, mz ) );
    var my = Normalize( Cross( mz, mx ) );
    var tx = Dot( mx, this.pos );
    var ty = Dot( my, this.pos );
    var tz = Dot( [-mz[0], -mz[1], -mz[2]], this.pos ); 
    var m = IdentM44();
    m[0]  = mx[0]; m[1]  = my[0]; m[2]  = mz[0]; m[3]  = 0;
    m[4]  = mx[1]; m[5]  = my[1]; m[6]  = mz[1]; m[7]  = 0;
    m[8]  = mx[2]; m[9]  = my[2]; m[10] = mz[2]; m[11] = 0;
    m[12] = tx;    m[13] = ty;    m[14] = tz;    m[15] = 1; 
    return m;
} 

var ShProg = {
Create: function (shaderList) {
    var shaderObjs = [];
    for (var i_sh = 0; i_sh < shaderList.length; ++i_sh) {
        var shderObj = this.Compile(shaderList[i_sh].source, shaderList[i_sh].stage);
        if (shderObj) shaderObjs.push(shderObj);
    }
    var prog = {}
    prog.progObj = this.Link(shaderObjs)
    if (prog.progObj) {
        prog.attrInx = {};
        var noOfAttributes = gl.getProgramParameter(prog.progObj, gl.ACTIVE_ATTRIBUTES);
        for (var i_n = 0; i_n < noOfAttributes; ++i_n) {
            var name = gl.getActiveAttrib(prog.progObj, i_n).name;
            prog.attrInx[name] = gl.getAttribLocation(prog.progObj, name);
        }
        prog.uniLoc = {};
        var noOfUniforms = gl.getProgramParameter(prog.progObj, gl.ACTIVE_UNIFORMS);
        for (var i_n = 0; i_n < noOfUniforms; ++i_n) {
            var name = gl.getActiveUniform(prog.progObj, i_n).name;
            prog.uniLoc[name] = gl.getUniformLocation(prog.progObj, name);
        }
    }
    return prog;
},
AttrI: function (prog, name) { return prog.attrInx[name]; },
UniformL: function (prog, name) { return prog.uniLoc[name]; },
Use: function (prog) { gl.useProgram(prog.progObj); },
SetI1: function (prog, name, val) { if (prog.uniLoc[name]) gl.uniform1i(prog.uniLoc[name], val); },
SetF1: function (prog, name, val) { if (prog.uniLoc[name]) gl.uniform1f(prog.uniLoc[name], val); },
SetF2: function (prog, name, arr) { if (prog.uniLoc[name]) gl.uniform2fv(prog.uniLoc[name], arr); },
SetF3: function (prog, name, arr) { if (prog.uniLoc[name]) gl.uniform3fv(prog.uniLoc[name], arr); },
SetF4: function (prog, name, arr) { if (prog.uniLoc[name]) gl.uniform4fv(prog.uniLoc[name], arr); },
SetM33: function (prog, name, mat) { if (prog.uniLoc[name]) gl.uniformMatrix3fv(prog.uniLoc[name], false, mat); },
SetM44: function (prog, name, mat) { if (prog.uniLoc[name]) gl.uniformMatrix4fv(prog.uniLoc[name], false, mat); },
Compile: function (source, shaderStage) {
    var shaderScript = document.getElementById(source);
    if (shaderScript)
        source = shaderScript.text;
    var shaderObj = gl.createShader(shaderStage);
    gl.shaderSource(shaderObj, source);
    gl.compileShader(shaderObj);
    var status = gl.getShaderParameter(shaderObj, gl.COMPILE_STATUS);
    if (!status) alert(gl.getShaderInfoLog(shaderObj));
    return status ? shaderObj : null;
},
Link: function (shaderObjs) {
    var prog = gl.createProgram();
    for (var i_sh = 0; i_sh < shaderObjs.length; ++i_sh)
        gl.attachShader(prog, shaderObjs[i_sh]);
    gl.linkProgram(prog);
    status = gl.getProgramParameter(prog, gl.LINK_STATUS);
    if ( !status ) alert(gl.getProgramInfoLog(prog));
    return status ? prog : null;
} };

var VertexBuffer = {
Create: function(attribs, indices, type) {
    var buffer = { buf: [], attr: [], inx: gl.createBuffer(), inxLen: indices.length, primitive_type: type ? type : gl.TRIANGLES };
    for (var i=0; i<attribs.length; ++i) {
        buffer.buf.push(gl.createBuffer());
        buffer.attr.push({ size : attribs[i].attrSize, loc : attribs[i].attrLoc, no_of: attribs[i].data.length/attribs[i].attrSize });
        gl.bindBuffer(gl.ARRAY_BUFFER, buffer.buf[i]);
        gl.bufferData(gl.ARRAY_BUFFER, new Float32Array( attribs[i].data ), gl.STATIC_DRAW);
    }
    gl.bindBuffer(gl.ARRAY_BUFFER, null);
    if ( buffer.inxLen > 0 ) {
        gl.bindBuffer(gl.ELEMENT_ARRAY_BUFFER, buffer.inx);
        gl.bufferData(gl.ELEMENT_ARRAY_BUFFER, new Uint16Array( indices ), gl.STATIC_DRAW);
        gl.bindBuffer(gl.ELEMENT_ARRAY_BUFFER, null);
    }
    return buffer;
},
Draw: function(bufObj) {
    for (var i=0; i<bufObj.buf.length; ++i) {
        gl.bindBuffer(gl.ARRAY_BUFFER, bufObj.buf[i]);
        gl.vertexAttribPointer(bufObj.attr[i].loc, bufObj.attr[i].size, gl.FLOAT, false, 0, 0);
        gl.enableVertexAttribArray( bufObj.attr[i].loc);
    }
    if ( bufObj.inxLen > 0 ) {
        gl.bindBuffer(gl.ELEMENT_ARRAY_BUFFER, bufObj.inx);
        gl.drawElements(bufObj.primitive_type, bufObj.inxLen, gl.UNSIGNED_SHORT, 0);
        gl.bindBuffer( gl.ELEMENT_ARRAY_BUFFER, null );
    }
    else
        gl.drawArrays(bufObj.primitive_type, 0, bufObj.attr[0].no_of );
    for (var i=0; i<bufObj.buf.length; ++i)
        gl.disableVertexAttribArray(bufObj.attr[i].loc);
    gl.bindBuffer( gl.ARRAY_BUFFER, null );
} };

var Texture = {};
Texture.HandleLoadedTexture2D = function( image, texture, flipY ) {
    gl.activeTexture( gl.TEXTURE0 );
    gl.bindTexture( gl.TEXTURE_2D, texture );
    gl.pixelStorei( gl.UNPACK_FLIP_Y_WEBGL, flipY != undefined && flipY == true );
    gl.texImage2D( gl.TEXTURE_2D, 0, gl.RGBA, gl.RGBA, gl.UNSIGNED_BYTE, image );
    gl.texParameteri( gl.TEXTURE_2D, gl.TEXTURE_MAG_FILTER, gl.LINEAR );
    gl.texParameteri( gl.TEXTURE_2D, gl.TEXTURE_MIN_FILTER, gl.LINEAR );
    gl.texParameteri( gl.TEXTURE_2D, gl.TEXTURE_WRAP_S, gl.REPEAT );
    gl.texParameteri( gl.TEXTURE_2D, gl.TEXTURE_WRAP_T, gl.REPEAT );
    gl.bindTexture( gl.TEXTURE_2D, null );
    return texture;
}
Texture.LoadTexture2D = function( name ) {
    var texture = gl.createTexture();
    texture.image = new Image(64,64);
    texture.image.setAttribute('crossorigin', 'anonymous');
    texture.image.onload = function () {
        var canvas = document.createElement( 'canvas' );
        canvas.width = 512;
        canvas.height = 256;
        var context = canvas.getContext( '2d' );
        context.drawImage( texture.image, 0, 0, canvas.width, canvas.height );
        Texture.HandleLoadedTexture2D( canvas, texture, true )
    }
    texture.image.src = name;
    return texture;
}
   
var ticks = 0;        
function drawScene(){

    var canvas = document.getElementById( "draw-canvas" );
    canvas.width = window.innerWidth;
    canvas.height = window.innerHeight;
    //canvas.width = 256;
    //canvas.height = 256;
    Camera.create();
    Camera.vp = [canvas.width, canvas.height];
    var currentTime = Date.now();   
    var deltaMS = currentTime - startTime;
    ticks = ticks + 1;
        
    gl.viewport( 0, 0, canvas.width, canvas.height );
    //gl.clearColor( 0.0, 0.0, 0.0, 0.0 );
    
    gl.clear( gl.DEPTH_BUFFER_BIT );
    gl.disable( gl.DEPTH_TEST );
    
    var texUnit = 1;
    gl.activeTexture( gl.TEXTURE0 + texUnit );
    gl.bindTexture( gl.TEXTURE_2D, textureObj );
    ShProg.Use( progBG );
    ShProg.SetI1( progBG, "u_texture", texUnit );
    
    var v_attr_inx = gl.getAttribLocation( progBG.progObj, "inPos" );
    gl.bindBuffer( gl.ARRAY_BUFFER, bufRect );
    gl.vertexAttribPointer( v_attr_inx, 2, gl.FLOAT, false, 0, 0 );
    gl.enableVertexAttribArray( v_attr_inx );
    gl.drawArrays( gl.TRIANGLE_FAN, 0, 4 );
    gl.disableVertexAttribArray( v_attr_inx );
    
    // set up draw shader
    ShProg.Use( progDraw );
    ShProg.SetM44( progDraw, "u_projectionMat44", Camera.Perspective() );
    ShProg.SetM44( progDraw, "u_viewMat44", Camera.LookAt() );
    var modelMat = IdentM44()
    modelMat = RotateAxis( modelMat, CalcAng( currentTime, 13.0 ), 0 );
    modelMat = RotateAxis( modelMat, CalcAng( currentTime, 17.0 ), 1 );
    ShProg.SetM44( progDraw, "u_modelMat44", modelMat );
    
    // draw scene
    gl.enable( gl.DEPTH_TEST );
    VertexBuffer.Draw( bufCube );
}

var startTime;
function Fract( val ) { 
    return val - Math.trunc( val );
}
function CalcAng( currentTime, interval ) {
    return Fract( (currentTime - startTime) / (1000*interval) ) * 2.0 * Math.PI;
}
function CalcMove( currentTime, interval, range ) {
    var pos = self.Fract( (currentTime - startTime) / (1000*interval) ) * 2.0
    var pos = pos < 1.0 ? pos : (2.0-pos)
    return range[0] + (range[1] - range[0]) * pos;
}    
function EllipticalPosition( a, b, angRag ) {
    var a_b = a * a - b * b
    var ea = (a_b <= 0) ? 0 : Math.sqrt( a_b );
    var eb = (a_b >= 0) ? 0 : Math.sqrt( -a_b );
    return [ a * Math.sin( angRag ) - ea, b * Math.cos( angRag ) - eb, 0 ];
}

var sliderScale = 100.0
var gl;
var progDraw, progBG;
var bufCube = {};
var bufRect = {};
function sceneStart() {

    var canvas = document.getElementById( "draw-canvas");
    canvas.width = window.innerWidth;
    canvas.height = window.innerHeight;
    var vp = [canvas.width, canvas.height];
    gl = canvas.getContext( "experimental-webgl", { alpha : true } );
    if ( !gl )
      return;

    progBG = ShProg.Create( 
      [ { source : "background-shader-vs", stage : gl.VERTEX_SHADER },
        { source : "background-shader-fs", stage : gl.FRAGMENT_SHADER }
      ] );
      progBG.inPos = gl.getAttribLocation( progBG.progObj, "inPos" );
    if ( progBG.progObj == 0 )
        return; 

    progDraw = ShProg.Create( 
      [ { source : "draw-shader-vs", stage : gl.VERTEX_SHADER },
        { source : "draw-shader-fs", stage : gl.FRAGMENT_SHADER }
      ] );
    progDraw.inPos = gl.getAttribLocation( progDraw.progObj, "inPos" );
    progDraw.inCol = gl.getAttribLocation( progDraw.progObj, "inCol" );
    if ( progDraw.progObj == 0 )
        return;   

    //textureObj = Texture.LoadTexture2D( "https://raw.githubusercontent.com/Rabbid76/graphics-snippets/master/resource/texture/supermario.jpg" );    
    textureObj = Texture.LoadTexture2D( "https://raw.githubusercontent.com/Rabbid76/graphics-snippets/master/resource/texture/background.jpg" ); 

    bufRect = gl.createBuffer()
    gl.bindBuffer( gl.ARRAY_BUFFER, bufRect );
    gl.bufferData( gl.ARRAY_BUFFER, new Float32Array( [ -1, -1, 1, -1, 1, 1, -1, 1 ] ), gl.STATIC_DRAW );

    // create cube
    var cubePos = [
      -1.0, -1.0,  1.0,  1.0, -1.0,  1.0,  1.0,  1.0,  1.0, -1.0,  1.0,  1.0,
      -1.0, -1.0, -1.0,  1.0, -1.0, -1.0,  1.0,  1.0, -1.0, -1.0,  1.0, -1.0 ];
    var cubeCol = [ 1.0, 0.0, 0.0, 1.0, 0.5, 0.0, 1.0, 0.0, 1.0, 1.0, 1.0, 0.0, 0.0, 1.0, 0.0, 0.0, 0.0, 1.0 ];
    var cubeHlpInx = [ 0, 1, 2, 3, 1, 5, 6, 2, 5, 4, 7, 6, 4, 0, 3, 7, 3, 2, 6, 7, 1, 0, 4, 5 ];  
    var cubePosData = [];
    for ( var i = 0; i < cubeHlpInx.length; ++ i ) {
      cubePosData.push( cubePos[cubeHlpInx[i]*3], cubePos[cubeHlpInx[i]*3+1], cubePos[cubeHlpInx[i]*3+2] );
    }
    var cubeNVData = [];
    for ( var i1 = 0; i1 < cubeHlpInx.length; i1 += 4 ) {
    var nv = [0, 0, 0];
    for ( i2 = 0; i2 < 4; ++ i2 ) {
        var i = i1 + i2;
        nv[0] += cubePosData[i*3]; nv[1] += cubePosData[i*3+1]; nv[2] += cubePosData[i*3+2];
    }
    for ( i2 = 0; i2 < 4; ++ i2 )
      cubeNVData.push( nv[0], nv[1], nv[2] );
    }
    var cubeColData = [];
    for ( var is = 0; is < 6; ++ is ) {
      for ( var ip = 0; ip < 4; ++ ip ) {
       cubeColData.push( cubeCol[is*3], cubeCol[is*3+1], cubeCol[is*3+2] ); 
      }
    }
    var cubeInxData = [];
    for ( var i = 0; i < cubeHlpInx.length; i += 4 ) {
      cubeInxData.push( i, i+1, i+2, i, i+2, i+3 );   
    }
    bufCube = VertexBuffer.Create(
    [ { data : cubePosData, attrSize : 3, attrLoc : progDraw.inPos },
      //{ data : cubeNVData,  attrSize : 3, attrLoc : progDraw.inNV },
      { data : cubeColData, attrSize : 3, attrLoc : progDraw.inCol } ],
      cubeInxData );

    startTime = Date.now();
    setInterval(drawScene, 50);
}

</script>
